The invention relates to technology for estimating Reference Signal Received Power (RSRP) in a cellular communication system, and more particularly to estimating RSRP in the presence of large Frequency errors.
Cellular communication systems typically comprise a land-based network that provides wireless coverage to mobile terminals that can continue to receive service while moving around within the networks coverage area. The term “cellular” derives from the fact that the entire coverage area is divided up into so-called “cells”, each of which is typically served by a particular radio transceiver station (or equivalent) associated with the land-based network. Such transceiver stations are often referred to as “base stations”. As the mobile device moves from one cell to another, the network hands over responsibility for serving the mobile device from the presently-serving cell to the “new” cell. In this way, the user of the mobile device experiences continuity of service without having to reestablish a connection to the network. FIG. 1 illustrates a cellular communication system providing a system coverage area 101 by means of a plurality of cells 103.
In some of the newer mobile cellular standards, such as the so-called Long Term Evolution (LTE) standard, transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) are employed. An LTE-compliant system uses OFDM as the multiple access technique (called OFDMA) in the downlink and is able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, very high data rates (e.g., up to 300 Mb/s in the downlink) are supported for the largest bandwidth. However, LTE is not limited to only high rate services, but is also usable for low rate services like voice. Since LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), Voice over IP (VoIP) is the service that carries speech.
There are several reasons why OFDM was chosen for the LTE system. One is that receiver complexity can be made relatively low. Another reason is that it, at least in theory, allows for very efficient usage of the available bandwidth. In case only one user is transmitting, it is possible to exploit that the channel quality typically is very different at different frequencies (in this respect, the channel is said to be “frequency selective”). Since the information in OFDM is transmitted on a large number of sub-carriers, different modulation and coding techniques can be applied on different sub-carriers, rather than using the same modulation and coding techniques on all sub-carriers. However, in order for this to be possible, the quality of the different sub-carriers of the channel (for instance the signal-to-noise-ratio) must be estimated and these estimates must be fed back to the transmitter.
In case several users are sharing the available bandwidth, the term orthogonal frequency division multiple access (OFDMA) is often used. In OFDMA, the sharing of the channel is achieved by allocating different sub-carriers to different users. The allocation of the sub-carriers to the different users can vary from one symbol to the next, so the channel is effectively divided in both time and frequency. Given a cellular system base station that serves several mobile stations, the channels from the base station to the different mobile stations vary differently and typically independently of one another. With respect to OFDMA, the idea of transmitting more information on the sub-carriers that have good quality, as described above, can be generalized in the following way. The quality on all sub-carriers for all users is determined. The base station then decides not only what the optimum modulation and coding techniques are, but also which sub-carriers should be allocated to which users.
An aspect of mobility in a cellular communication system is selection of a cell from a number of candidate cells, the selected cell being one that can best serve the User Equipment (UE). Measurement of the received signal strength from cells in the vicinity of the unity are fundamental for a proper working cell selection mechanism. RSRP measurements are used for this purpose.
RSRP measurement methodology is also defined by the system specifications, and in this regard 3GPP TS 36.214 version 8.6.0 Release 8 (2009-04) states that RSRP measurements should be estimated using Cell-specific Reference Symbols (CRS) on port 0 and possibly also on port 1 if port 1 is being used by eNodeB. The RSRP is typically computed as
                              RSRP          =                      P            -            bias                          ⁢                                  ⁢        where                            (        1        )                                P        =                              1            M                    ⁢                                    ∑                              m                =                1                            M                        ⁢                                                                                                1                                          N                      S                                                        ⁢                                                            ∑                                                                        (                                                      k                            ,                            l                                                    )                                                ∈                                                  S                          m                                                                                      ⁢                                          H                                              k                        ,                        l                                                                                                                        2                                                          (        2        )                                RSSI        =                              1            M                    ⁢                      1                          N              S                                ⁢                                    ∑                              m                =                1                            M                        ⁢                                          ∑                                                      (                                          k                      ,                      l                                        )                                    ∈                                      S                    m                                                              ⁢                                                                                      H                                          k                      ,                      l                                                                                        2                                                                        (        3        )                                bias        =                                                            N                S                            -              1                                      N              S                                ⁢                      (                          RSSI              -              P                        )                                              (        4        )            where Hk, l is a channel estimate obtained for OFDM symbol l, and frequency index k by derotating the received symbols in the frequency domain for port 0 (i.e., by multiplying the received (rotated) pilot symbol by the conjugate of the known pilot symbol). The set Sm is a set of NS CRS symbols (each CRS symbol being located at a particular frequency in a given symbol) over which the derotated symbols are averaged in order to reduce noise. This technique is sometimes also referred to as “coherent summation”. The sets Sm are enumerated using the index m. The set Sm typically consists of all CRS symbols in a Resource Block (RB) and the number of sets Sm could depend on the bandwidth. As a nonlimiting example, one could use M=6 for a bandwidth of 1.4 MHz.
The inventor of the subject matter defined herein has recognized a set of circumstances that make measuring RSRP problematic. In particular, it can be seen from the above equations that the RSRP is a function of terms that are themselves at least partly a function of frequency. However, when measuring RSRP on cells that are not part of the set of serving cells of the UE, frequency errors may arise due to timing errors resulting from the clocks between cells not being synchronized and lack of processing time in the UE to accurately measure the propagation delay profile of neighboring cells (as used herein, the term “timing errors” means the inability to accurately estimate the first path of the propagation channel that is to be measured, regardless of reason for that inability).
In the calculation of RSRP indicated above, the term that is especially sensitive to frequency and timing errors is the one that accumulates coherently the derotated CRS, that is the term
      1          N      S        ⁢            ∑                        (                      k            ,            l                    )                ∈                  S          m                      ⁢                  H                  k          ,          l                    .      
It is possible to estimate the frequency error with good accuracy, but this conventionally involves receiving a fair amount of pilots (i.e., CRS's). Such extensive measurement usually takes longer than what a typical UE would want to allocate for measurement purposes.
The problem is even more acute for measurement on frequencies that are not used for data reception because it is desired to spend as little time as possible doing measurements on other frequencies, since time spent on other frequencies means less time for receiving data on the camping frequency.
It is therefore desirable to provide technology (e.g., methods and apparatuses) that enable measurement of RSRP relatively quickly and with good accuracy even in the presence of significant errors between eNodeB clocks, Doppler effects, and errors in determining the shape of the power delay profile (used to identify the start location of an OFDM symbol).